It is often desirable to make clear aqueous compositions that incorporate organic components that are either insoluble or only slightly soluble in water. The problem is ubiquitous in formulating, for example, certain pharmaceutical or flavor & fragrance applications. Some related art teachings refer to such aqueous compositions as solutions, while others describe them as transparent oil-in-water microemulsions. Either way, the goal is essentially the same: to solubilize organic compounds in water to obtain a clear mixture.
Many solubilizers have been identified. In one well-known approach, a water-miscible co-solvent (ethanol, acetone, propylene glycol, dimethylsulfoxide, glycol ethers, or the like) is simply added. For one example, see U.S. Pat. No. 5,081,104. Unfortunately, even traces of co-solvents in the final product are often unacceptable.
Inclusion complexes with cyclodextrins (see, e.g., U.S. Pat. No. 4,596,795) are another way to solubilize hydrophobic components. However, these require tedious research to identify cyclodextrins that are just the right size to accept the hydrophobic substrates, and they also require that the inclusion complex be water soluble. Moreover, because of its high molecular weight, a large proportion of cyclodextrin is normally needed.
Other solubilizers for making clear aqueous compositions that contain a hydrophobic component include: surfactants or mixtures of surfactants (see, e.g., U.S. Pat. No. 5,374,614 or 5,320,863); epoxide-glycol reaction products (U.S. Pat. No. 4,299,737); ester reaction products of a C1-C4 alcohol and a C8-C16 alkylene-substituted succinic acid derivative (German Pat. DE 2 915 948); monoalkyl citrates (U.S. Pat. No. 5,635,190); and combinations of a surfactant and a solubilizer, which may be a C2-C7 dicarboxylic acid or a salt thereof (U.S. Pat. Appl. Pub. No. 2006/0084589). None of these references discloses clear aqueous compositions that contain a monoester salt of the type described and claimed herein.
Salts of monoesters derived from a saturated C6-C20 alcohol and a saturated dicarboxylic acid (e.g., glutaric acid, succinic acid, adipic acid) are generally known. They have been taught for use in making bakery products (U.S. Pat. No. 3,762,932), bar soaps (Canadian Pat. No. 1,329,104), and concentrated liquid laundry detergents containing less than 50 wt. % water (U.S. Pat. No. 4,714,565). In the flavor & fragrance industry, such salts have been recognized as physiological coolants that can be used at part-per-million levels in such aqueous compositions as mouthwashes and beverages (see U.S. Pat. Nos. 5,725,865; 6,121,315; and 6,365,215). None of the references teaches to use at least 2 wt. % of the monoester salt to solubilize a hydrophobic organic compound in an aqueous composition that contains at least 55 wt. % water.
In sum, numerous industries would benefit from the availability of more ways to solubilize hydrophobic organic compounds in water to provide clear aqueous compositions. In particular, the pharmaceutical and flavor & fragrance industries are always looking for better ways to solubilize vitamins, physiological cooling or warming agents, medicines, dietary supplements, cosmetics, flavors, fragrances, or combinations of these, in aqueous media. Preferred compositions would avoid co-solvents or cyclodextrins. Ideally, the compositions would be clear, stable, economical, and easy to formulate.